1. Field of the Invention
The present invention relates to an optical scanner. More particularly, the present invention relates to an optical scanner having a latched carrier capable of sliding along a guide rail.
2. Description of the Related Art
Most optical scanners operate by projecting light from a light source onto a scan document so that the reflected light is transmitted via a set of reflecting mirrors and a lens to a charge couple device (CCD). Finally, the image picked up by the CCD is converted into digital signals for further reading. Together, the set of reflecting mirrors, the lens and the optical sensor (that is, the charge couple device) are commonly referred to as an optical system.
Aside from lining the optical sensor in one horizontal direction, most optical scanners have a carrier with an optical system thereon. Through the sliding motion of the carrier, the optical system moves (along a vertical direction) to carry out a document scanning operation and obtain a two-dimensional planar image.
In a conventional scanner, a guide rail passes through the carrier so that the carrier is free to move along the guide rail. However, this type of structural design often leads to a number of problems that are described in more detail below. FIG. 1 is a top view (wit the top casing removed) of a conventional optical scanner. FIG. 2 is a front view of the scanner in FIG. 1. FIG. 3 is a cross-sectional view of the scanner in FIG. 1.
As shown in FIG. 1, the carrier 102 of a conventional scanner 100 has a pair of axle sheaths 104. The passage of a guiding rail 106 through the axle sheaths 104 of the carrier 102 constrains the carrier 102 to move along the guiding rail 106. In general, the guiding rail 106 is fabricated using metal. An optical system (not shown) is mounted on the carrier 102. Here, the upper portion of the casing 108b is removed to display the relationship between the carrier 102 and the guiding rail 106 of the optical scanner 100 within the lower casing 108a. 
In FIG. 2, the guiding rail 106 is attached to base stands located within the lower casing 108a of the optical scanner 100. To fix the guiding rail 106 onto the pair of base stands, a hole is drilled in each of the base stand 120. Thereafter, one end of the guiding rail 106 is fastened to one base stand 120 via a rivet 110a while the other end of the guiding rail 106 is fastened to another base stand 120 using a screw 110b. 
Transmission of the carrier 102 is achieved through an electric motor 112 driving a transmission belt 114. The carrier 102 is able to move through the action of the belt 114 linked to a latch 116 on one side of the carrier 102. On the other end of the carrier 102 (the far side of the guiding rail 106), movement is supported by a roller 118 pinned to the carrier 102.
However, the structural layout of the aforementioned optical scanner has the following drawbacks.
1. Demand on assembling tolerance leading to an increase in labor hour: the base stands 120 in the lower casing 108a need to be drilled and the ends of the guiding rail 106 need to be fastened to the respective base stands 120. Hence, the number of steps for assembling the guiding rail 106 is increased in addition to consider a tolerance between the two.
2. The guiding rail may drop off during site tests: because only one end of the guiding rail 106 is firmly tightened. Hence, an environmental testing (such s shock or drop test) may cause the guiding rail 106 to drop.
3. A constant distance of separation between the scan document and the optical system is hard to maintain: because the carrier 102 is fastened to the base stands 120 in the lower casing 108a while the document is placed on a platform above the upper casing 108b. Hence, the assembly in the upper casing 108b will directly affect the distance of separation between the optical system on the carrier 102 and the scan document.
In brief, major drawbacks of a conventional optical scanner includes: more tolerance-demand for assembly, large number of assembling steps, possible dropping of the guiding rail after a site test and difficulty in maintaining a constant separation between the optical system and the scan document.